1. Field of the Invention
The present invention relates to a hybrid optical integrated circuit in which an optical waveguide and an active optical device such as a light source, a light detector and so on are mounted on the same substrate and a method for fabricating such hybrid optical integrated circuits.
2. Description of the Prior Art
As a result of the recent development of optical communications systems, various optical components such as optical branching and mixing circuits, wavelength demultiplexers and multiplexers and so on are required to be mass-produced and supplied at a low cost. So far the optical components are of a bulk type comprising in combination a prism, a lens, a filter and so on, but they have poor productivity because it takes a long time to assemble and adjust the optical components. As a result, they are expensive and it is difficult to make them compact in size and light in weight. Therefore, the development of optical communications systems into various fields has been adversely impeded.
In order to overcome the above-described problems, various attempts have been made to provide planar waveguide type optical devices in the form of an optical integrated circuit, but in this case it is extremely difficult to couple an optical waveguide whose film thickness is of the order of one micron to an optical fiber. Therefore, such attempts have remained as a dim idea and so far no satisfactory means for fabricating such optical integrated circuits has been proposed.
In view of the above, attempts have been made to combine a miniaturized prism or lens with a light source and a detector to form an integral module, thereby providing an optical integrated circuit which is compact in size and inexpensive to manufacture, but the prism and lens have no waveguide structure so that in assembly they must be aligned with each other with a higher degree of accuracy. Thus, the above-described problems have not been overcome yet.
Practical optical integrated circuits may be generally divided into a monolithic structure and a hybrid structure. The monolithic structure is such that all the required optical devices such as light emitting devices, optical waveguides and light detectors are made of the same material (for instance, InGaAsP series) and fabricated on the same substrate. The hybrid structure is such that light emitting devices and light detectors are mounted on a substrate upon which an optical waveguide is formed, whereby an optical integrated circuit is provided. For instance, an optical waveguide is formed on a substrate and a light source and a light detector (for instance, InP series or GaAs series semiconductor devices) are disposed at the ends of the optical waveguide, respectively. Such hybrid optical integrated circuits as described above have a great advantage in that optical waveguides can be made of a material having an extremely low degree of loss (such as high-silica glass). On the other hand, in the case of the monolithic optical integrated circuits, the optical waveguides are made of a semiconductor material. But, such a semiconductor material has a high degree of absorption loss. Furthermore, a thick film waveguide adapted for use in multi-mode optical circuits cannot be made of a semiconductor material. As a consequence, only the hybrid optical integrated circuits are feasible at present.
Attempts at fabricating hybrid optical integrated circuits by incorporating active optical devices such as light sources and light detectors on the substrate on which a low-loss optical waveguide is formed have been made for a long time, but they have remained as a dim idea. Thus, there have not been provided hybrid optical integrated circuits which can be practically and satisfactorily used in optical communications.
In order to fabricate the hybrid optical integrated circuit, the following three steps are required:
(1) the first step in which an optical waveguide is formed on a substrate;
(2) the second step in which optical fibers and active optical devices are disposed at predetermined positions at the ends of the optical waveguide; and
(3) the third step in which power supply lead wires or the like are connected to the active optical devices.
For instance, Japanese Patent Application Laid-open No. 57-84189 disclosed a hybrid optical integrated circuit in which, in order to satisfy the above-described first and second steps, optical fibers and other optical devices are disposed in a groove formed in the surface of a substrate. In this case, in order to align the groove with an optical waveguide formed on the substrate, a mask alignment step is required. However, the optical waveguide is raised by a height of several tens of micron meters above the surface of the substrate, so that a mask cannot be placed close to the surface of the substrate. As a result, the mask alignment is extremely difficult.
Y. Yamada et al. disclosed in "Optical-fiber coupling to high-silica channel waveguides with fibreguiding grooves", Electronics Letters, 12th April, 1984, Vol. 20, No. 8, pp. 313-314 that a planar waveguide of SiO.sub.2 --TiO.sub.2 is formed on a silica glass substrate and then a channel waveguide and guiding grooves for guiding an optical fiber are simultaneously formed by reactive ion etching (RIE) technique.
Furthermore, Y. Yamada et al. disclosed in "Fabrication of a high silica glass waveguide optical accessor", Electronics Letters, 5th July, 1984, Vol. 20, No. 14, pp. 589-591 an optical accessor in which a SiO.sub.2 --TiO.sub.2 planar waveguide is formed on a silica glass substrate and then a waveguide having input and output ports and branching and mixing ports and guiding grooves for guiding optical fibers are simultaneously formed by the RIE technique.
As disclosed in the above-described papers, the connection method in which guides for aligning optical fibers are formed simultaneously with the patterning of an optical circuit on a high-silica glass optical waveguide and the optical waveguide and the optical fiber are connected by utilizing such guides ensures.a highefficiency connection between the optical fiber and the optical waveguide without the need of cutting and polishing the ends of the optical waveguide and without the need of alignment between the optical fiber and the optical waveguide.
However, when an optical fiber having an outer diameter of 125 .mu.m and a core diameter of 50 .mu.m is connected to an optical waveguide, the high-silica glass optical waveguide must be etched to a depth of about 90 .mu.m to accommodate the optical fiber. In the etching process, amorphous Si (a-Si) is used as a mask and the mixture gas consisting of C.sub.2 F.sub.6 and C.sub.2 H.sub.4 is used as an etchant to perform reactive ion etching. However, when such an etching method is used to etch a groove to a depth as deep as 90 .mu.m, a long etching time is needed and hence there arises another problem that the width of the optical waveguide thus formed is considerably narrower than the width of the pattern used in the photomasking step. In order to prevent the decrease in pattern width, so far an optical circuit has been etched to a depth of about 70 .mu.m and an optical fiber whose one end is so etched that the clad has an outer diameter of about 70 .mu.m is inserted into guiding grooves to perform the connection.
When silica glass is used as a substrate, the coefficient of thermal expansion of a glass film deposited on the surface of the substrate is higher than that of silica, so that the glass film is subjected to tensile stress with respect to the substrate and consequently the glass film is likely to be cracked. Therefore, the composition of the glass film must be selected to prevent cracking.
Furthermore, in the hybrid optical integrated circuit of the type described above, spatial wiring using gold wires must be employed in order to supply power to active optical devices. As a result, in the case of a hybrid optical integrated circuit which has a relatively large chip area as compared with electronic integrated circuits, the length of the gold wires is more than a few millimeters, so that there arises a reliability problem because of instability resulting from mechanical vibrations.
In order to overcome the above-described problem, it would be considered to provide a pattern of electrical conduction paths over the surface of a substrate, but, as described hereinbefore, the top surfaces of the optical waveguide and the guides are as high as several tens of micron meters above the surface of the substrate, so that there arises the problem that it is extremely difficult to carry out the photolithographic process for forming the pattern of electrical conduction paths, including the step of coating photoresist. It may be proposed to form a pattern of electrical conduction paths prior to the formation of an optical waveguide film over the surface of the substrate, but there arises again the problem that the underlying pattern is broken when the high-silica optical waveguide is formed at such a high temperature above 1200.degree. C.
Meanwhile, in order to provide a hybrid optical integrated circuit, it is required that an optical waveguide and optical components are coupled to each other on the same substrate. In this case, if the size of a light spot of the optical waveguide is largely different from that of a light spot of a light emitting device, a lens must be interposed between the optical waveguide and the light emitting device so as to convert the size of the light spot, thereby increasing the coupling efficiency. However, there arises the problem that it is extremely difficult to optically align the optical waveguide, the lens and the light emitting device when they are disposed on the same substrate. As a result, there has not been provided yet a hybrid optical integrated circuit having a light emitting device and an optical waveguide which are coupled to each other with a high degree of coupling efficiency as described above.
In the hybrid optical integrated circuit, in order to couple an optical waveguide to a light source such as a semiconductor laser, light-emitting diode and a light detector such as a photodiode or the like with a high degree of efficiency, there has been proposed a method in which optical fibers are interposed between the optical waveguide on the one hand and the light source and the light detector on the other hand. According to this method, one end of the optical fiber is connected to one end of the optical waveguide, while the other end of the optical fiber is connected to the light source or light detector. However, according to this method, the optical fiber of 10 cm through 1 m in length is extended between the optical waveguide on the one hand and the light source or light detector on the other hand, so that the optical waveguide device cannot be made compact in size. Especially, in the field of optical information processing, the time delay of light caused by the optical fiber of 10 cm-1 m in length is disadvantageous in that the information processing speed by optical devices is limited.
In order to overcome the above-described problem, there has been proposed a method in which a light source and a light detector are directly coupled to the ends of an optical waveguide, respectively.
FIGS. 1A and 1B are used to explain a prior art example of this coupling method. Reference numeral 101 designates a silica glass substrate; 102, an optical waveguide; and 103, a semiconductor laser. FIG. 1A shows an example in which the semiconductor laser 103 and the optical waveguide 102 are coupled to each other by utilizing one end surface 101a of the substrate 101, while FIG. 1B shows an example in which the optical waveguide 102 and the semiconductor laser 103 are optically coupled to each other at a suitable position on the same substrate 101 without being limited to the end surface 101a of the substrate 101.
In the coupling method as shown in FIG. 1A, the semiconductor laser 103 can be replaced with a light emitting diode or a light detecting photodiode. This coupling method is very simple, but has a defect that the coupling point is limited only to the end surface 101a of the substrate so that some problem or limitation of design arises when an optical circuit is designed.
According to the method as shown in FIG. 1B, the semiconductor laser 103 is disposed on the substrate 101 at any desired position, but there arises the problem that it would be difficult to couple a flat optical component having a flat light emitting or receiving surface (as shown in FIG. 2) such as a light-emitting diode or a photodiode instead of the semiconductor laser 103 shown in FIG. 1B to the optical waveguide. FIG. 2 shows an example of the structure of a light emitting or receiving device having a flat light emitting or receiving surface. The light emitting or receiving device comprises a semiconductor substrate 121 of 1 mm.times.1 mm and a light receiving or emitting surface 122 of 100 .mu.m in diameter. It is impossible to couple such light emitting or receiving device as shown in FIG. 2 to the end of the optical waveguide 102 having a height of about several .mu.m through 100 .mu.m on the substrate 101 as shown in FIG. 1B. That is, the method as shown in FIG. 1B may be applied only to a device such as a semiconductor laser which emits or receive the light in a direction which is parallel with the surfaces of a semiconductor substrate, but cannot be applied to an optical device which emits or receives the light in the direction perpendicular to the surface of the semiconductor substrate.